In the end, an enlarger is just a projector. You need a light source that's uniform across at least the area of the negative (usually condensing lenses or a diffuser, often with a center grad ND filter, are used to get this), a way to hold the negative in the light and keep it flat, a lens that will cover the negative (that is, the corners of the negative should still be illuminated evenly and be sharp on the work surface), a means of focusing, a means of adjusting the size of the projected image, and a flat place to put the enlarging paper.
Ideally, the lens should have an aperture iris, and camera lenses will work (though enlarging or "process" lenses from the printing trade are better optimized for short focal distance, the difference only shows if you examine the print with a loupe or print really large), providing you can lock the shutter open. My only lens for 6x6 format is a taking lens off a Kodak Reflex II TLR; it works fine at any size I'm likely to print. Focusing need not be a bellows -- a box that slides into another box will work fine, as long as it keeps the lens plane parallel to the negative and base board. Most enlargers move the head up and down to adjust image size, but you can enlarge horizontally, too (Ansel Adams did it this way for decades, from 8x10).
If you're a bit handy, you can build a 4x5 enlarger for about the cost of a used one that will need lots of dusting and cleaning (and possibly repairs). Negative carriers can be cut from mat board (commonly used for mounting prints for framing). So can easels (which hold the paper flat and delimit the exposed area, so as to leave a white border). Alternatively, if you're very lucky, you may find an enlarger with negative carriers and lenses for carting it away.
As noted, a 4x5 camera and a copy stand can do most of this -- you'll need a way to mount the negative so light can shine though it, and you'll probably want a ground glass that's uniform and doesn't have the clipped corners commonly seen on view cameras.